renderer: make default frame mixer consistent

Unlike mpv, which has an explicit --interpolation option to enable frame
mixing, our frame mixing is "always on" by default, whenenever you use
the pl_render_frame_mix API (which is used in both vf_libplacebo and
plplay, to name at least two). So we want to avoid pulling in something
with subjective appearance significantly different from the "nearest
neighbour" default.

Oversampling here is fine, since it basically just fixes what is
arguably suboptimal usage anyway, and the alternative is awful judder
from 3:2 pulldown.

Fixes: https://code.videolan.org/videolan/libplacebo/-/issues/308

src/renderer.c

@@ -212,7 +212,7 @@ const struct pl_render_params pl_render_high_quality_params = {
     PL_RENDER_DEFAULTS
     .upscaler           = &pl_filter_ewa_lanczossharp,
     .downscaler         = &pl_filter_hermite,
-    .frame_mixer        = &pl_filter_hermite,
+    .frame_mixer        = &pl_filter_oversample,
     .sigmoid_params     = &pl_sigmoid_default_params,
     .peak_detect_params = &pl_peak_detect_high_quality_params,
     .color_map_params   = &pl_color_map_high_quality_params,

src/tests/options.c

@@ -53,9 +53,9 @@ int main()
     REQUIRE_CMP(*(int *) data->value, =, pl_render_default_params.tile_size, "d");
     REQUIRE_STREQ(data->text, "32");
 
-    const char *hq_opts = "upscaler=ewa_lanczossharp,downscaler=hermite,frame_mixer=hermite,deband=yes,sigmoid=yes,peak_detect=yes,peak_percentile=99.99500274658203,contrast_recovery=0.30000001192092896,dither=yes";
+    const char *hq_opts = "upscaler=ewa_lanczossharp,downscaler=hermite,frame_mixer=oversample,deband=yes,sigmoid=yes,peak_detect=yes,peak_percentile=99.99500274658203,contrast_recovery=0.30000001192092896,dither=yes";
     // fallback can produce different precision
-    const char *hq_opts2 = "upscaler=ewa_lanczossharp,downscaler=hermite,frame_mixer=hermite,deband=yes,sigmoid=yes,peak_detect=yes,peak_percentile=99.99500274658203125,contrast_recovery=0.30000001192092896,dither=yes";
+    const char *hq_opts2 = "upscaler=ewa_lanczossharp,downscaler=hermite,frame_mixer=oversample,deband=yes,sigmoid=yes,peak_detect=yes,peak_percentile=99.99500274658203125,contrast_recovery=0.30000001192092896,dither=yes";
 
     pl_options_reset(test, &pl_render_high_quality_params);
     const char *opts = pl_options_save(test);